The rule of thumb is that it costs $220-$260 per square foot to renovate and combine a property. It seems like you have a wonderful and very unique property, so perhaps the post renovation will be attractive. What you need to really consider is how much more your monthly maintenance will increase by. When units are combine, the cost of maintenance is typically more than what you would pay for an similar size apartment in your building. This above market maintenance can be 20-30% more than the average. Another thing to consider is who the potential buyer would be post combination. If you will be living there yourself, great, but if you're combining it for the hopes of flipping it to someone else, there are a few things that must be considered. First and foremost, there is a good chance that someone looking to buy a 2-3 bedroom typically will have children. If your apartment is a 3rd floor walkup, people with children tend to not give consideration to walk up buildings. The thought of having to bring a children's stroller up and down 3 flights of stairs is very daunting and not attractive. So it really comes down to how many bedrooms you have now versus the potential combo design, while taking into consideration the final maintenance (combined unit's maintenance). Overall, the price per square foot post combination can potentially be 10-25% more than if sold separately. Full disclosure: I am a real estate agent and have gone through one personal renovation in the city and have represented a couple that purchased two units and have since combined. They are happy, but for resale value, the maintenance is going to be a tough sell. The average coop monthly maintenance charges averaged $1.74 per square foot, compared to condominium monthly common charges of $1.71, however for combined coop apartments, the average probable increases to $1.90 or more. Regarding outdoor space, if it is useable and depending on its attractiveness, many appraisers take approximately 50% of the exterior square footage and add it to the interior. I just happened to see this posting, so I realize it is a bit past due, but if you have any questions or concerns, please let me know (email@example.com). I hope I was somewhat helpful.